From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933039Ab0I0NnG (ORCPT ); Mon, 27 Sep 2010 09:43:06 -0400 Received: from mtagate1.de.ibm.com ([195.212.17.161]:46606 "EHLO mtagate1.de.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755850Ab0I0NnE (ORCPT ); Mon, 27 Sep 2010 09:43:04 -0400 Date: Mon, 27 Sep 2010 15:42:57 +0200 From: Martin Schwidefsky To: Oleg Nesterov Cc: Michael Holzheu , Shailabh Nagar , Andrew Morton , Venkatesh Pallipadi , Peter Zijlstra , Suresh Siddha , John stultz , Thomas Gleixner , Balbir Singh , Ingo Molnar , Heiko Carstens , linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [RFC][PATCH 09/10] taskstats: Fix exit CPU time accounting Message-ID: <20100927154257.7910cda3@mschwide.boeblingen.de.ibm.com> In-Reply-To: <20100926181127.GA26985@redhat.com> References: <1285249681.1837.28.camel@holzheu-laptop> <1285250541.1837.95.camel@holzheu-laptop> <20100923171025.GA26623@redhat.com> <1285330688.2179.305.camel@holzheu-laptop> <20100926181127.GA26985@redhat.com> Organization: IBM Corporation X-Mailer: Claws Mail 3.7.6 (GTK+ 2.20.1; i486-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, 26 Sep 2010 20:11:27 +0200 Oleg Nesterov wrote: > Hi, > > On 09/24, Michael Holzheu wrote: > > > > On Thu, 2010-09-23 at 19:10 +0200, Oleg Nesterov wrote: > > > > > > On 09/23, Michael Holzheu wrote: > > > > > > > > Currently there are code pathes (e.g. for kthreads) where the consumed > > > > CPU time is not accounted to the parents cumulative counters. > > > > > > Could you explain more? > > > > I think one place was "khelper" (kmod.c). It is created with > > kernel_thread() and it exits without having accounted the times with > > sys_wait() to the parent's ctimes > > No. Well yes, it is not accounted, but this is not because it is > kthread. We noticed that behavior with kernel threads but as you point out the problem is bigger than that. > To simplify the discussion, lets talk about utime/cutime only, > and lets forget about the multithreading. > > It is very simple, currently linux accounts the exiting task's > utime and adds its to ->cutime _only_ if parent does do_wait(). > If parent ignores SIGCHLD, the child reaps itself and it is not > accounted. > > I do not know why it was done this way, but I'm afraid we can't > change this historical behaviour. Why? I would consider it to be a BUG() that the time is not accounted. Independent of the fact that a parent wants to see the SIGCHLD and the exit status of its child the process time of the child should be accounted, no? And I'm not a particular fan of the "this has always been that way" reasoning. > > Ok, the problem is that I did not consider exiting threads that are no > > thread group leaders. When they exit the ctime of the parent is not > > updated. Instead the time is accumulated in the signal struct. > > I think I am a bit confused, but see above. With or without threads > the whole process can exit without accounting. Got the part about self-reaping processes. But there is another issue: consider an exiting thread where the group leader is still active. The time for the thread will be added to the utime/stime fields in the signal structure. Taskstats will happily ignore that time while the group leader is still running. Please keep in mind that we want to get to a point where it is possible to get a 100% coverage of cpu cycles in the last snapshot cycle through the taskstats interface. Otherwise the precise top would not be very precise .. -- blue skies, Martin. "Reality continues to ruin my life." - Calvin.